Set screws are used in many ways in order to lock one element of a device relative to another. Set screws are quite important in the art of medical implants, as it is often necessary to capture one element of the implant relative to another and to then lock the two relative to one another to prevent subsequent movement therebetween. Failure to properly lock two elements of a medical implant together may result in failure of the implant and possible serious injury to the patient within which the implant is placed.
For instance, orthopedic injuries, deformities, and degenerative diseases often require intervention in the form of surgery for placing implants to stabilize an internal structure, promote healing, and relieve pain. In the area of spinal surgery, for example, a common procedure includes placement of bone securing elements in the form of screws or hooks that are joined by a connecting rod spanning between these elements. Once placed, the rod must be firmly secured to the bone securing elements to provide a stable construct which effectively immobilizes a corresponding portion of the spine. For this, large forces are applied to the construct typically in the form of a set screw or locking element which presses firmly against the rod to secure the rod to the bone securing element (e.g., pedicle screw or other anchor element).
Additionally, it is generally desirable for these spinal implants to maintain a small profile so as to minimize the impact upon the patient. The loading required to lock the components can cause component deformation in these low profile implants, resulting in assembly loosening and possible implant failure. Specifically, when spinal loading occurs postoperatively, the walls of the securing element trapping the rods may be forced outward causing the rod to be released and allowing room for play in the rod.
As an example configuration of a vertebral stabilization implant device, anchors (e.g., pedicle screws) are connected to the vertebrae and are connected to one another by a connecting means, such as a brace or rod. For instance, a first pedicle screw may be coupled to a first vertebral level and a second pedicle screw may be coupled to another vertebral level, and a distraction rod is used to connect the first and second pedicle screws. The head of the pedicle screw (or a receiving member coupled thereto) is typically fork-shaped. With annular pedicle screw heads a distraction rod is guided through and is fixed on both sides of the head with the help of a nut. With fork-shaped heads, inner (female) threaded sections are included within the heads for receiving a set screw for applying direct or indirect pressure on a previously inserted distraction rod for securing such distraction rod relative to the pedicle screws. A similar receiving part for a distraction rod or likewise is also known for hook-like retaining components, for example with so-called lamina hooks or pedicle hooks which are hooked into the corresponding vertebrae parts.
Conventional set screws for use in medical implant devices typically utilize threads which are referred to as unified threads, which have a V-shaped cross-section. That is, the edges of the unified thread's cross-section form a V shape. V-shaped threads work reasonably well in devices where a bore is provided that completely surrounds the set screw and has a mating thread that mates with the thread of the set screw. However, many medical implants, such as open-headed (or “open-back”) bone screws have a receiving member (or “rod cage”) coupled to the pedicle screw head, which does not provide for a bore that will entirely encircle the set screw. For example, the above-mentioned fork-shaped receiving member typically has a channel formed by a plurality of noncontiguous (or discontinuous) walls that include an inner (female) thread that forms a helical spiral about a center longitudinal axis of the channel. That is, the walls forming the channel have a noncontiguous diameter. Such receiving member, in certain implementations, effectively provide a cylindrical sleeve that has a longitudinal slit in one or more planes for at least part of its length.
In such implementations, the set screw is inserted into the channel (or sleeve) of such fork-shaped receiving member. Accordingly, in this type of implant device, the set screw also functions as a closure member and spans between a pair of discontinuous threaded surfaces. When V-shape threaded set screws are utilized for this purpose, the forces exerted by the set screw during torquing are partially parallel to the axis of rotation of the set screw and partially radially extending outwardly from the set screw. The radial outward forces can and frequently do spread the arms (or noncontiguous “walls”) of the receiving member within which the set screw is being torqued sufficiently to allow for failure of the set screw. Other example types of set screws proposed for use with such noncontiguous wall implementations (or “open-back configurations”) for functioning as a closure member that spans between a pair of noncontiguous threaded surfaces within a medical implant device are described in U.S. Pat. No. 6,454,768 to Jackson, U.S. Pat. No. 6,074,391 to Metz-Stavenhagen et al., and U.S. Pat. No. 6,296,642 to Morrison et al.
A further difficulty that has been experienced with such noncontiguous wall implementations as the above fork-shaped receiving member configurations is that the upright legs or wall sections of the set screw receiving member can experience splaying after implantation. For example, in the spinal field, after a rod is placed into the channel in the body portion of an open-back spinal fixation element, a closure or locking element is typically engaged in the body portion over the rod to clamp it within the channel so that there is no relative movement between the rod and the fixation element. Since no relative motion is possible, stresses placed on the rod after implantation are transmitted via the fixation element to the bone. In some cases, these stresses cause the legs or wall sections of the fixation element (such as the fork-shaped receiving member mentioned above) on either side of the slot to splay or move away from each other. Significant splaying of the fixation element generally results in its failure, since the closure or locking element will loosen its clamping of the rod. When that happens, the rod is free to move with respect to the fixation element, and may become disconnected with the fixation element altogether. In such a case, the therapeutic value of the implant is obviated, and further injury or complications may also result.
To prevent splaying of the noncontiguous walls, prior medical implant devices have included a nut, cap, clamp or similar apparatus to surround and hold the walls of the fixation element together. For example, in U.S. Pat. No. 5,672,176 to Biedermann et al., a rod is placed into a slot in the fixation element, the locking member is engaged with the fixation element to press down via an intermediary part on the rod, and an outer nut is threaded on the outside of the fixation element. Although effective in controlling splaying, these devices have tended to be relatively more expensive and less efficient to implant compared with devices without an outer nut or cap. The outer nut or cap also adds to the profile of the medical implant device, making the device more difficult to implant in the frequently limited area in which to perform surgery and/or place an implant. A larger implant can also result in a higher risk of residual pain to the patient or potential complications.
There is therefore a need remaining in the industry for medical implant devices, and particularly orthopedic devices, which minimize the profile and bulk of the components of the device and minimizes the cost and difficulty of using such devices, while still preventing splaying of the noncontiguous walls of fixation elements. For instance, it is desirable to have a closure member, such as a set screw, that is configured to aid in preventing (rather than urging) the opposing walls of an implant from pulling away from the closure member.